The SMB is permanently free, it's not a Trial service. It has some functionality limitations compared to the paid versions, but most small installs should find the features available more than sufficient.
Regarding Trunk, can you advice which trunk provider you chose, and is this the first trunk you are adding? In general for SMB the trunk name needs to truly be unique globally, not just within your instance. So if you just named it as trunk1 or the name of the sip-trunk provider, that's probably taken.